88 PROGRESS IN MICROSCOPY 



Colour phase-contrast microscopes 



F. Zernike, who invented phase-contrast microscopy, was also 

 the first to investigate colour phase-contrast potentiahties. Making 

 use of the dispersive properties of thick phase-plates is the basis of 

 the method. It was thoroughly studied by C. P. Saylor, A. T. Brice 

 and F. Zernike. They use a phase-ring set on one of the objective 

 lenses. The space not taken up by the ring is coated with a thin layer 

 of another substance. These two substances have the same index in 

 the spectrum centre (0 555 n) but different dispersion. In the violet, 

 the phase-ring refraction index is lower than the adjacent layers and 

 higher in the red. Therefore the positive or negative phase-contrast 

 is the same for the selfsame object according as to whether the specimen 

 is illuminated by either of the two spectral areas spaced apart by 

 a 0555 [x wave-length. Let us consider, for example, an object the 

 refraction index of which exceeds that of the ambient medium: it will 

 be brighter or darker than the remainder of the field in red and blue 

 light respectively. In white light the object is yellow-orange, edged 

 with bluish purple. 



Barer has shown a type of colour phase-contrast based on the 

 use of dichroic filters. 



Locquin's Varicolor yields a coloured phase-contrast. All that 

 is required is to illuminate the specimen with two suitably combined 

 colours to obtain a positive phase-contrast image of one of the 

 colours and a negative phase-contrast image of the other. 



In Frangon's colour phase-contrast, a quartz plate cut 90 from 

 the axis (not shown in Fig. 2.21) is inserted between the polarizer Py 

 and the phase-plate Q. The quartz plate is 1 -87 mm thick and causes 

 a 45° rotation of the yellow. If the polarizer is 45° from the incidence 

 plane, the yellow is overshadowed through glass reflection (Brewster's 

 law) whereas such is not the case in the metal-reflected diffracted light. 

 Various combinations are feasible by altering the condenser orien- 

 tation or the quartz thickness. 



Grigg devised a colour phase-contrast microscope featuring an 

 objective fitted with an ordinary phase-plate: he substituted the con- 

 denser diaphragm for two suitably coloured filters. The device is an 

 extension of Rheinberg's conventional system to phase-contrast. 

 Barham suggested using two phase rings illuminated by two radiations. 

 The observed image is the outcome of superimposing two differently- 

 coloured phase-contrasted images. 



